OBJECTIVE: To understand the health needs of the population living in Mexico's marginalized urban areas. METHODS: A population-based survey of families residing in poor, urban neighborhoods, in five geographic areas in Mexico (northern, central, southern, south-east, and Mexico City), selected through multistage sampling. Interviews were conducted and anthropometric measurements were taken in the home, and included all members of the participating family. Analysis was carried out on positive health factors, nutrition, reproductive health, health problems, and mental health, and results were disaggregated by age and sex. RESULTS: In all, 24 707 individuals participated. The interviewees were found to have minimal schooling (6 years or less). Only 46.8% had health care coverage, be it public or private. Among the children, 19.8% were malnourished; overweight was prevalent from adolescence onwards. Of adolescents 12-19 years of age, 15.7% were sexually active, but only 57.7% of the males and 41.9% of the females in this age group were using some method of contraception. Of the adults, 5.9% suffered from diabetes and 11.5%, from high blood pressure. In the sample of adolescents, adults, and elderly adults, tobacco use was 21.2%; alcohol consumption, 36.0%; illicit drug use, 9.5%; and depression, 20.2%. CONCLUSIONS: The health needs of people living in Mexico's marginalized urban areas proliferate in the context of a young population with weak ties and little support from family and health services. They face crisis and disease-infant malnutrition, high-risk pregnancy, and addictions-the byproducts of disparities in social progress. The rate of chronic conditions was similar to that of the general population of Mexico.